Production of metal powder



Jan. 30, 1945A.

R. L. PATTERSON PRODUCTION OF METAL POWDER Filed April 15, 1943 BRAS/vfmvo /fro/v off/DE /FO/V POM/HER YNVENTOR ATTORN EY Patented Jan. 30,1945` NTED STATES PATENT Ol=l=lclel Y 2,368,489 PRODUCTION OF METALPOWDER Raymondl L. Patterson, No# York, N. Y.

Appliostlon April 15, 194s, serial No. 4s3,14c 4 claims. lol. 'l5-34)IIhis invention relates to the production of metal powders and providesimprovements which are applicable to the production of ferrous andnon-ferrous metal powders by gaseous reduction of finely-dividedcompounds of the metals without substantial fusion. The invention isconcerned primarily with the production of metal powders from metallicgrindings and the like, but may also be applied with advantage toimprove powders produced from other materials, for example scale, shot,spatter, machinings, turnings, sawlngs, Iborings, etc. of iron or steel,and iron oxide.

In the machining, and especially in the grinding or fillingv of steeland iron, substantial quantities of finely-divided metallic grindingsare produced. These grindings generally are contaminated with abrasivessuch as silica, aluminum Oxide or silicon carbide, together with oxidesof the metallic constituents, and hence are unsuitable for use as'metalpowders or even as melting stock, and in large part they have beenwasted heretofore. Such grindings have represented an inexpensivepotential source of iron powder, but

the recovery of the metallic iron from such grind- 25 ings in acondition such that it is suitable for powder metallurgical applications(i. e. for the manufacture of objects by compression and sintering ofthe powders) heretofore has presented a vexing problem. v

As the result of my investigations, I have developed improvements whichare especially applicable to the recovery as high grade iron powder ofthe iron content of iron and steel grindings and similar nely-dividedproducts containing some reduced metal accompanied by compounds thereof.However, the process is applicable to the y treatment of other types ofgrindings, including r brass and bronze grindings, and may also beemployed to produce metal powders from nnelydivided compounds thereof(for example, oxides of iron, nickel, cobalt, tungsten, molybdenum,copper, etc.) by gaseous reduction in the solidstate.

To summarize, in the production of metal powders by gaseous reduction ofa finely-divided compound of the metal in the solid state, i. e. withoutsubstantial fusion, my invention contemplates the improvement whichcomprises sub# proportion of gas becomes occluded, adsorbed or otherwiseentrapped on or near the surfaces of the partially reduced particles andmasks them so that the gaseous reducing agent .does not have anopportunity to operate. The scrubbing or abrasion treatment releases thegases on or near the particle surfaces and also loosens non-metallicsolids adhering thereto, so that both can be removed, leaving cleansurfaces which are susceptible to the action of the gaseous reducingagent.

I have also discovered that, in the case of iron powders produced bygaseous reduction, it is desirable to conduct the reduction in at leasttwo stages, in the first of which a substantial proportion of the ironcan be reduced at a relatively low temperature and such thatsubstantially no fritting together of the particles occurs. In thesecond stage, the reduction can be carried out at a much highertemperature, i. e. at a temperature above that at which fritting wouldtend to begin in the first stage.

As applied to the production of iron powder, therefore, my inventioncontemplates a primary gaseous reduction stage preferably although notnecessarily conducted at a relatively low temperature, say, -in theneighborhood of 600 C. to 700 C. This preliminary reduction operation isfollowed by a scrubbing treatment wherein gases,

0 with or without unreduoed metallic compounds.

jecting the metal powder after or during a pre- 5o liminary gaseousreduction operation to a scrubbing treatment and subjecting the scrubbedmetal powder to a. second reduction treatment in the solid state. Itappears that, in the gaseous reduction o! metal oxides and the like, asubstantial 55 are removed-from the surfaces of the particles. Thescrubbed particles are then subjected to a second gaseous reductiontreatment, preferably but not necessarily conducted at a temperaturehigher than that in the first stage, i. e. at a temperature in excess of800 C.

To consider my invention in somewhat greater detail. I have discoveredthat metal powders produced by gaseous reduction of finely-dividedmetallic compounds can be improved (in the sense that stronger compactscan be made from the powder by compression and sintering)l if the metalpowder resulting from a primary gaseous reduction operation in the solidstate is subjected to an abrasion or scrubbing treatment to clean offthe powder particle surfaces and remove entrapped or occluded gases, theabrasion treatment being followed by a, second gaseous reductiontreatment to remove additional oxide lms, etc. As indicated above, theprocess is applicable to a, variety of powders, including iron powdersmade by gaseous reduction O1' iron oxide, say, iron scale or ironpowders produced from iron grindings. Thus, iron powder made by gaseousreduction of iinely-divided iron or steel scale may be improved markedlyby subjecting it to an abrasion treatment followedby a second reductiontreatment. For example, a commercial iron powder produced by gaseousreduction of scale in a single treatment, upon compression at 50 tonsper square inch and sintering in hydrogen for 2 hrs. at 1000 C. foi-ms acompact having a tensile strength of 17,300 lbs. per square inch with anelongation of about 4% in 2 inches. When this same iron powder issubjected to an abrasion treatment followed by a second reductiontreatment in hydrogen. the same powder metallurgical procedure resultsin the formation of a compact having a tensile strength of 30,000 lbs.per square inch with an elongation of 10% in 2 inches.

Although some improvement can be obtained merely by the second reductiontreatment without the intermediate abrasion step, the results whenabrasion precedes the secondary reduction operation are much superior.

The improved results set forth above for iron powder are in largemeasure duplicated when the process is applied to other materials. forexample bronze powders derived from grindings.

One of the primary dimculties heretofore experienced in the gaseousreduction oi finely-divided iron oxide in the solid state to form ironpowder has been the tendency of the iron oxideiron powder mixture tofrit during the reduction operation. Numerous attempts have been made toovercome the fritting diiiiculties through the use of complex furnacestructures and the like, but this has added complexity to the operationwithout, in all cases, providing a remedy. I have discovered that thefritting dimculties heretofore encountered in reducing iron oxide andother iron compounds in the solid state at a high temperature can beavoided, at least to the extent which permits a practicable commercialoperation by conducting the reduction operation in a plurality of steps,with a preliminary reduction operation preferably conducted at a lowtemperature. say, 600 C. to 700 C. and preferably in the neighborhood of670 C. Thereafter, and following the abrasion treatment, it is subjectedto a second gaseous reduction treatment, preferably at a hightemperature, say, 800 C. to 1000 C. (a temperature oi.' about 850 C.being particularly effective) without danger of excessive fritting. Inthe preliminary reduction operation a substantial part of the compoundof the metal (say more than half) should be reduced.

One of the principal advantages of the multil.stage reduction treatment,particularly when the reducing agent is hydrogen, arises from the factthat the tendency for fritting is low when the ratio of reducing gas tothe gaseous product of reduction is high. In single stage treatments,the gaseous product of combustion, say water vapor, tends to reach highconcentrations in some zones in a furnace, say in zones in which the gasilow tends to be somewhat stagnant. In the multistage operation, theproportion o f reducing gas to reduction products can be kept at shigher level, with resultant decrease in the tendency toward fritting.Moreover, when the powder from one stage is removed from the furnaceatmosphere and also preferably cooled in transfer to the second stage,it appears that occluded gases and the like in or on the particles tendto be expelled at least in part. the expulsion being aided by scratchingthe particle surfaces in the scrubbing or abrasion operation.

:,scacea These and other aspects of my invention will be more thoroughlyunderstood in the light of the accompanying flow sheet and the followingdetailed description of the process of my invention as applied to theproduction of iron powder from iron or 'steel grindings.

Referring to the now sheet, it will be observed that the raw materialfrom which the iron powder is produced consists of illings or grindingswhich are formed in iron or stee1 finishing operations. In the case ofsteel grindings, both high and low carbon steels are usually includedand the combined carbon content of the grindings. i. e. the carbonpresent as iron carbide, may range from 0.1% to about 1%. In someinstances, the grindings may be 0f Dure iron rather than of steel and insuch case will contain little or no carbon. The grindings, dependingupon the material from which they are derived, may include various alloyingredients, including chromium, nickel, molybdenum. tungsten, vanadium,etc. Generally speaking, the total metallic iron content of the filingsor grindings ranges from about 82% to 86%. In addition, the grindingsusually contain a substantial proportion of non-metallic abrasive powder(silica, aluminum oxide. silicon carbide, etc.) derived from the wheelor other apparatus used to produce the grindings, together withconsiderable iron oxide (say 5% to 10%). The iron oxide in the grindingsis generally the result of oxidation of the iron or steel duringgrinding, which usually is conducted in an oxidizing atmosphere andgenerates substantial heat. The oxide may be present in the form ofcoatings on the particles of grindings or as small discrete particles.

Depending upon the operation in which the grindings are produced, theymay or may not be contaminated with organic material or water. Commonly,the grindings contain small quantities of coolauts or lubricants s uchas oils and soaps.

The particle size of the grindings varies over a broad rang. Some of theparticles may be as coarse as 6 mesh while others are in the form ofimpalpable powder. In one operation a typical screen analysis ci' thegrindings was as follows:

` Mean As will be seen from the ilow sheet, the first operation in thetreatment oi' ferrous grindings is a mild comminution adapted to breakup aggregates. In many instances the requisite comminution can beobtained merely by screening, which also serves to separate largeforeign bodies and metal particles that are too coarse to be included inthe nal product.

Following the primary comminution or screening operation, the grindingsare subjected to primary cleaning adapted to remove some of the abrasiveand iron oxide. 'I'he primary cleaning preferably is conducted with amagnetic separator, although other methods may be employed for removingnon-metallic impurities. These methods include elecrostatic separation.pneumatic separation and tabling.

Employing a. magnetic separator, a substantial proportion of theabrasive is removed and the The grindings from the primary cleaningoperation are sent to a primary reduction operation adapted to softenthe metal present in the grind# ings, reduce metallic compounds on thesurface of the particles and, if desired, remove carbon at the sainetime. A variety of types of equipment for gaseous reduction oflnely-divided material may .be employed. I prefer to employ a furnace ofthe type described and claimed in United States Patent No. 2,267,041 andprovided with a porous hearth. In this furnace, the grindings aretreated in moist hydrogen at a relatively low temperature, say, in theneighborhood of 670 C. for a short time, say, minutes. Such treatmentbrings about a reduction in carbon content from, say, .90% to .37% andat the same time (by re- Aducing oxides present in the grindings) raisesthe metallic iron content from Although I prefer to employ wet hydrogenfor simultaneous decarburizing and reducing, other gaseous reducingagents may be employed. Among such reducing agents are carbon monoxide,hydrocarbon gases such as methane, and mixtures of gaseous carbonaceousreducing agents or hydrogen with inert gases such as nitrogen. By way ofexample, a mixture of 80% nitrogen with hydrogen is suitable forbringing about simultaneous reduction and decarburization. If highconcentrations of carbonaceous gases, such as CO, are employed, theamount of decarburization which occurs may be slight. Moreover,carbonaceous gases and -particularly zsm-91% to hydrocarbons should bediluted with a substantial proportion of inert gases or non-carbonaceousreducing gases in order to prevent the precipitation of soot in thepowder.

Following the primary reduction treatment, the partially reducedgrindings are subjected to a scrubbing or abrasion treatment adapted toremove both solid and gaseous impurities from the surface of theparticles. Thus, the grindings from the iirst reduction treatment may besubjected to mild comminution in a hammer mill or other equipmentadapted to abrade substanttially all particles, thus removing from thebody of the surface of the particles non-metallic solid impurities andoccluded, absorbed or entrapped gases which blanket the metal surfaceand prevent further decarburizing or reduction. At the same timeunreduced metallic-oxides are exposed. The abrasion treatment should -beconducted carefully and in equipment designed to clean the powderparticles without flattening them substantially. Although rolls or a:ball -mill may be employed for abrading the grindings, superior.

results are obtained with a hammer type of mill, for example a Mikropulverizer made 'by the Pulverizing Machinery Corporation, which servesto expose any unreduced oxide on the particles and to break down anyfilms that tend to retard reduction of the oxide within the particles,without bringing about excessive work-hardening of the metal or atteningthe particles to the extent that they cause laminations in compacts madefrom the powder product. There is, of course,

some reduction in particle size, but this should be as little aspossible consistent with proper cleaning. An amount of particle sizereduction which has given satisfactory result in the abra- 2,868,489metallic iron content of the grindings is raised sion treatment isillustrated by the following Screen analyses:

Screen analyses of grindings I After Alter Mesh (Tyler) primaryreduction abrasion grindings are subjected toa secondary reductiontreatment which may also include decarburizing. This treatmentconveniently is conducted in a rotary kiln or a vibrating hearth furnacebut with the same gaseous reducing agents used in primary reduction andfor about the same time. If wet hydrogen is used in the rst treatmentany hydrogen is preferred for the second. However, the secondaryreduction treatment differs from the primary in that it preferably isconducted at a higher temperature (say, in the range of 800 C. to 1000C.,a temperature of about 850 C. being Vparticularly effective), andwith substantially dryer hydrogen, so that decarburization does not takeplace at the expense of reduction.

The secondary reduction operation raises the metallic iron content ofthe grindings to about 97 %99% and, depending upon the nature of thegaseous reducing agent, may also bring about a substantialdecarburization'. Thus, in the case of moist, i. e. commercial hydrogen,the carbon content of the grindings maybe reduced from about .37% toabout .03%. nitrogen with 20% moist. hydrogen -results in a reduction ofcarbon content from about .37% to about .07%.

Following the secondary reduction operation,I

the grindings are screened or otherwisesubjected to mild comminution tobreakup aggregates and the resulting powder is suitable for-use inpowder metallurgy.

To consider certain specific examples of the practice of the invention,the raw material was steel grindings which, as received, contained82.86% Fe, 0.80% C; .20% Ni and .30% Mn together with 12% ofnon-metallics, including unreduced oxides, abrasives, lubricants, etc./These grindings, afterscreening to break up aggregates and removecoarse pieces Weresubjected to electromagnetic separation, which raisedits iron content to 86.5%. Thereafter, it was treated in a rabbletypefurnace for approximately 15 minutes in commercial hydrogen at atemperature ofv about 675 C.

The product of the primary reduction treatment contained about 96% ironand 0.37% car- The powder had a compression` ratio of 3.25 and thesintered compacts made from it had tensile.

strengths of about 20,000 p. s. i. with elongations of 6% in 1 inch. Theproduct was, therefore.

fairly satisfactory from the powder metallurgical The use of a mixtureof standpoint, but was greatly improved by further treatment, i. e.abrasion followed by a second rcduction treatment.

Samples of the grindings from the primary heat treatment were scrubbedin a hammer mill and were then subjected to reduction treatmentemploying diilerent agents. The three types ot reduction treatment andthe results, including the subsequent powder metallurgical tests aresummarized as follows:

Secondary reduction treatments Temp. 860 880 Duration minutes.-

Reducing agent Anal sis product:

er cent Fs Per cent C 83% NI m% H-.

16; Natural gas.

Com act formation:

ompressive force t. s. i-. 50 Sintering temp. Bintering time..T hours..2 Compression rat1o Physical iproperties:

Tens e strength-compact p. s. i. Eiiclinglation of compact-per cent Asindicated hereinbefore, the process is also applicable to the treatmentof non-ferrous grind- 4ings, for example brass or bronze grindings. In

such instance magnetic separation for removal of the non-metallica 4isnot feasible but electrostatic separation Works well. However, ferrousimpurities. such as chips of iron and steel, can be removed by magneticseparation and in some in-l stances it is advisable to employ bothmagnetic and electrostatic separation.

The abradng treatment followed by the secondary reduction treatment mayalso be employed for improving the powder metallurgical characteristicsoi' powders produced by gaseous reduction from scale, shot, spatter,machinings, turnings, etc. as well as from ores, precipitates, calcines,etc. The improvement in the powders thus brought about is substantiallyas great as that obtained in powder derived from grindings.

I claim:

1. In the production of iron` powders by gaseous reduction of anely-divided solid compound of iron, the improvement which comprisesconducting the reduction in a plurality oi stages, in one of which asubstantial proportion of the iron is reduced to a metallic state at arelatively low temperature below that at which substantial frit`.

ting tends to occur, a later reduction'stage being conducted at atemperature substantially above that at which tritting would begin totake place .in the earlier stage but without bringing about substantialiritting in this later reduction stage.

C. and above that at which fritting would begin to take place in theearlier stage.

3. In the production of iron powders by gaseous reduction of afinely-divided solid compound of iron, the improvement which comprisesconducting the reduction in a plurality of stages, in the first of whicha substantial proportion of the iron is reduced to a metallic state `ata temperature in the neighborhood of 670 C., a later re duction stagebeing conducted without substantial fritting but at a temperature in theneighborhood of 850 C.

4. In the treatment of ferrous grindings to produce iron powder suitablefor powder metallurgical purposes, the improvement which comprisessubjecting the grindings to a primary cleaning operation to remove from.the grindings abrasive and iron oxide, subjecting the cleaned grindingsto a primary reduction treatment in va gaseous atmosphere at, atemperature in the neighborhood of 670 C., subjecting the reducedgrlndings to abrasion by mild comminution to remove gases andnon-metallic solids from the particle surfaces of the grindings,subjecting the

